1. Field of the Invention
The present invention relates to a variable resistor for use in, for example, commercial equipment such as hearing aids, measuring instruments, communication devices, and sensors, and more particularly, to a small variable resistor.
2. Description of the Related Art
As a known example of such a variable resistor, Japanese Examined Patent Application Publication No. 5-59561 discloses a variable resistor wherein a resistor substrate on which terminals are mounted is insert-molded into a resin case, a rotor on which a slider is installed and an O-ring are accommodated in the case, the rotor is prevented from rising or moving by placing a metallic cover on the case, and the cover is prevented from slipping off by engaging protrusions provided on the sides of the case with holes in leg portions provided on the cover.
As another known example of such a variable resistor, Japanese Unexamined Patent Application Publication No. 5-3108 discloses a variable resistor wherein a metallic cover is placed on a case from the upper portion thereof, and leg portions protruding from the cover are folded inwardly along the bottom surface of the case.
In both variable resistors, the rising and moving of the rotor relative to the case due to the elastic forces of an O-ring and a slider is prevented by using the metal cover.
In each of these variable resistors, the resistor substrate is insert-molded into the case to provide heat resistance and superior sealing characteristics when the variable resistor is soldered to a printed circuit board. Hence, the resistor substrate (particularly, in the case of a ceramic substrate) is prone to cracking during molding, or molding resin may flow onto the surface of the substrate on which the resistor is provided, resulting in the formation of an insulating film thereon. Furthermore, in these variable resistors, it is necessary to take special steps to prevent the resin from intruding into the inside of the substrate, in order to prevent any insulating film from being located on the surface of the substrate. These problems result in reduced productivity and increased cost.
Further, a tool engagement groove is provided in the upper surface of the rotor, and the resistance value is adjusted by rotating the rotor using a tool, such as a screwdriver, that is engaged with the tool engagement groove. However, since it is necessary to decide the location of a start point and an end point of the rotor, a stopper mechanism must be provided. In general, for variable resistors, stopper protrusions which are in contact with each other are provided on the external surface of the rotor and on the internal surface of the case, and the rotational angle of the rotor is controlled so as to be within a certain range. However, this makes the molding complicated, and the protruding portion of the rotor and the protruding portion of the case interfere with each other, and accordingly the assembly of the rotor becomes very difficult.
In response to this problem, a variable resistor having a protrusion that is integrally formed at the inner edge of an opening of a metal cover and is arranged to strike against a protrusion disposed on the upper surface of the rotor so as to control the rotational angle of the rotor, has been proposed.
However, in small variable resistors having an approximate size of about 2 mm to about 3 mm, the constituent parts also become small and thin, and accordingly it becomes difficult to obtain sufficient mechanical strength of the parts. Particularly, the thickness of metal parts becomes very small, and when the protrusion of the metal cover abuts against the protruding portion of the rotor, the metal cover is deformed and the rotation of the rotor may not be able to be sufficiently controlled.
To overcome the above-described problems, preferred embodiments of the present invention provide a variable resistor which reliably seals the space between a rotor and a substrate without insert-molding the substrate into a case, and which resistor is produced at a greatly reduced cost.
Furthermore, preferred embodiments of the present invention provide a simple and inexpensive variable resistor in which, when a stopper mechanism is provided between a metal cover and a rotor, a strong metal cover is obtained and the rotational angle of the rotor is reliably controlled.
Also, preferred embodiments of the present invention provide a variable resistor including a case which is open at the top and the bottom, a substrate which is fitted into the lower opening of the case, the substrate including a collector electrode on the top surface thereof and an arcuate resistor provided around said collector electrode, a rotor rotatably fitted into the upper opening of the case, a slider mounted on the bottom surface of the rotor and making sliding contact with the collector electrode and the resistor, an annular packing member disposed between the rotor and the substrate for sealing the space therebetween, and a metallic cover having a top plate portion for supporting the top surface of the rotor, a hole provided at the top plate portion such that a portion of the rotor is exposed, and a pair of leg portions extending downward along the sides of the case. The metallic cover supports the bottom surface of the substrate such that the metallic cover is disposed on the case from the upper portion of the case and the leg portions thereof are folded inwardly along the bottom surface of the case.
When assembling this variable resistor, first, the substrate is fitted into the lower opening of the case, and then the rotor is fitted into the upper opening of the case. It is preferable that the annular packing member be disposed on the top surface of the substrate and the slider be mounted on the bottom surface of the rotor in advance. Next, when putting the metallic cover on the case from the upper portion of the case, the pair of leg portions extend downwardly along the sides of the case. In this situation, a portion of the rotor is exposed from the window hole. The leg portions are folded inwardly along the bottom surface of the case and support the bottom surface of the substrate by the tip portions thereof. Thereby, the rotor is prevented from rising, the substrate is prevented from slipping off from the case, and the packing member and the slider are sandwiched between the rotor and the substrate. That is, a closed space is provided between the rotor and the substrate. By disposing the slider, the resistor, and the collector electrode, within this space, the intrusion of moisture and solder flux from the outside is prevented, which produces a variable resistor that achieves very stable performance.
Preferably, a spacer portion to maintain a desired spacing between the rotor and the substrate is provided on the inner surface of the case. Thereby, variations in assembly are avoided, and the compression allowance between the slider and the packing in the assembling process is uniform, and hence the electrical characteristics and the sealing characteristics of this variable resistor are very stable and uniform.
Furthermore, it is preferable that the packing member be directly applied on the top surface of the substrate, and on the outer peripheral side of the resistor. The packing member may instead be provided separately from the substrate and the rotor, and the packing may be disposed therebetween. However, in this case the packing member is prone to cause positional deviations, and it is difficult to maintain stable sealing characteristics. In contrast, the direct application of the packing onto the top surface of the substrate reliably prevents positional deviations thereof.
Moreover, guide grooves for guiding the leg portions of the metallic cover are preferably provided on the outer side-surfaces of the case. This stabilizes the positioning of the cover on the case, and facilitates the assembly thereof.
According to another preferred embodiment of the present invention, a variable resistor includes a case which is open at a top surface and a bottom surface thereof, a substrate fitted into the lower opening of the case, the substrate including a collector electrode on the top surface thereof and an arcuate resistor disposed around the collector electrode, a rotor rotatably fitted into the top opening of the case, the rotor having a tool engagement groove at an upper surface thereof, an annular groove disposed around the tool engagement groove at the upper surface thereof, and a first stopper portion provided inside the annular groove, a slider mounted on the bottom surface of the rotor, the slider including a first contact arm arranged to contact with the collector electrode and a second contact arm arranged to contact with the arcuate resistor, the first and second contact arms being integral with the slider, a metallic cover having a top plate portion supporting the top surface of the rotor, a hole provided at the top plate portion such that the tool engagement groove of the rotor is exposed, a pair of leg portions extending downwardly along two sides of the case, and a second stopper portion arranged to protrude in the inner edge of the hole of the metallic cover, the second stopper portion being bent downward to be inserted into the annular groove of the rotor so as to control the rotational angle of the rotor by abutting the first stopper portion, wherein the metallic cover supports the bottom surface of the substrate such that the metallic cover is placed on the case from the upper portion of the case and the leg portions thereof are folded inwardly along the bottom surface of the case.
The substrate is fitted into the lower opening of the case, the rotor having the slider attached thereto is accommodated in the case from above, the metallic cover is placed on the case from above, and the lower surface of the substrate is supported by bending inwardly the leg portions of the metallic cover on the bottom surface of the case. Thus, the top plate portion of the metallic cover prevents rising of the rotor, and at the same time, the substrate is prevented from falling off. In this way, it is not necessary to fix the substrate to the case in advance. Also, since the case, the rotor, and the substrate are held together by the metallic cover, the assembly is simple, and moreover, since the shape of the case is simplified, the manufacturing cost is reduced.
The annular groove is preferably located in the upper surface of the rotor, and the second stopper portion of the metallic cover, which is bent downward in the inner edge of the hole of the metallic cover, is inserted into the annular groove of the rotor. When the rotor is rotated, the first stopper portion of the rotor disposed inside the annular groove abuts against the second stopper portion of the metallic cover, and then the rotor stops. At this time, if the rotor is strongly rotated, a bending stress acts on the second stopper portion and it may be possible that the rotation of the rotor cannot be controlled due to deformation of the second stopper portion. However, since the second stopper portion is inserted in the annular groove of the rotor, the bending deformation in the thickness direction of the second stopper portion is controlled by the annular groove. Dropping off of the second stopper portion of the metallic cover from the first stopper portion of the rotor is prevented. Therefore, even if the metallic cover is made of a thin metal, the rotational angle of the rotor is reliably controlled.
According to a preferred embodiment of the present invention, a burring portion curved towards the annular groove of the rotor is provided in the inner edge of the hole of the metallic cover, and the upper surface of the rotor may be located under the top plate portion of the metallic cover. That is, when the burring portion is provided and the upper surface of the rotor is located lower than the top plate portion of the metallic cover, the burring portion functions as a guide when a screwdriver is inserted, and the lead to the tool engagement groove is improved. Furthermore, the burring portion improves the strength of the top plate portion of the metallic cover and has the advantage of reinforcing the second stopper portion.
According to preferred embodiments of the present invention, it is preferable that the second stopper portion of the metallic cover is provided in the vicinity of the leg portions. If the second stopper portion is provided in the vicinity of the leg portions, when the rotation of the rotor is controlled by the second stopper portion engaged in the rotor, the rigidity against deformation of the cover is further improved. Even if the metallic cover is made of a thin metal, the rotational angle of the rotor is effectively controlled.
The features, characteristics, elements and advantages of the present invention will be clear from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings.